602.2 Construction. Plenum enclosures shall be constructed of materials permitted for the type of construction classification of the building.
The use of gypsum boards to form plenums shall be limited to systems where the air temperatures do not exceed 125°F (52°C) and the building and mechanical system design condidons are such that the gypsum board surface temperature will be maintained above the airstream dew-point temperature. Air plenums formed by gypsum boards shall not be incorporated in air-handling systems utilizing evaporative coolers.
:•A building of noncombustible construction as defined in the IBC is permitted to have only noncombustible
plenums constructed of noncombustible materials. The intent of requiring noncombustible plenums in buildings of noncombustible construction is to maintain consistency between the materials used in plenum construction and the materials used in building construction. Materials classified as noncombustible in the IBC are used to construct the plenum. These are materials that have been tested to the requirements of ASTM E 136.
In general, noncombustible plenums are those formed and bounded by noncombustible materials and contain few, if any, combustible materials within the plenum space. These plenums represent a relatively low risk for fire spread and smoke generation because of the limited amount of combustible materials present. Although some combustible materials may be exposed within the plenum space, the types and amounts of these materials are strictly limited by Section 602.2.1. The area of a noncombustible plenum is limited only by the size of the fire area containing it.
A building of combustible construction is permitted by the IBC to use combustible or noncombustible building materials for specified components of the structure. In these buildings, either noncombustible plenums or combustible plenums can be used. Again, the intent is to require that the plenum be constructed of or formed by materials that are consistent with the materials allowed for building construction.
A combustible plenum may be constructed using any approved combustible or noncombustible material that is permitted for combustible construction in accordance with the IBC. The flame spread and smoke-developed indexes of all materials within a combustible plenum must also comply with the applicable sections of the IBC. Combustible materials, such as ducts, pipe, wire, tubing and insulation coverings may be exposed within the plenum only in accordance with Section 602.2.1.
Note, however, that in buildings that are not permitted to have concealed spaces, the use of a plenum is
not feasible. See the IBC for the definitions of the different construction types.
The airstream and surface temperature restrictions on the use of gypsum boards in plenums are intended to prevent premature deterioration of the gypsum board because of condensation on the plenum walls or excessive drying of the gypsum board (see commentary, Section 603.5.1).
602.2.1 Materials within plenums. Except as required by Sections 602.2.1.1 through 602.2.1.5, materials within plenums shall be noncombustible or shall have a flame spread index of not more than 25 and a smoke-developed index of not more than 50 when tested in accordance with ASTM E 84.
Exceptions:
1. Rigid and flexible ducts and connectors shall conform to Section 603.
2. Duct coverings, linings, tape and connectors shall conform to Sections 603 and 604.
3. This section shall not apply to materials exposed within plenums in one- and two-family dwellings.
4. This section shall not apply to smoke detectors.
5. Combustible materials enclosed in noncumbustible raceways or enclosures, approved gypsum board assemblies or enclosed in materials listed and labeled for such application.
❖ Materials located within a plenum, regardless of whether the plenum is constructed of or bounded by combustible or noncombustible materials, must be noncombustible or must have a flame spread index of 25 or less and a smoke-developed index of 50 or less when tested in accordance with ASTM E 84.
This section addresses those items that are installed within plenums, not the materials that bound and create the plenum space (see Section 602.2).
The code recognizes that the plenum space is often used to accommodate components of other building systems, such as electrical, plumbing, fire protection, communication and mechanical. For this reason, the code permits exposure of limited types of combustible materials used for these systems within the plenum. A flame spread index of 25 or less and a smoke-developed index of 50 or less are viewed as acceptable values for combustibles in a plenum because of the minimal risk associated with those materials. Any hazard associated with smoke production in a plenum is also reduced by the air handler shutdown requirements of Section 606.
Previous editions of the code used the term "exposed within plenums" when addressing this issue. However, some designers and installers used that language to install plastic pipe and other combustible material with some insulation wrapped around it, claiming that the material was no longer exposed. If the wrapping material were to become damaged, loosened or destroyed in a fire, the combustible material could be exposed to the fire and produce hazardous smoke that
would be spread to other parts of the building through the plenum. The word "exposed" was deleted in the 2006 edition of the code to close this loophole. Note that Exception 5 still allows electrical wiring and cable to be installed in a plenum when enclosed in noncombustible conduit.
There is no stated limit on the aggregate amount of these combustible materials that may be exposed in a plenum. For example, a test of electrical wiring using the procedures of UL 910 accounts only for the amount of wiring present within that tested assembly. Once installed in the plenum, however, additional combustible materials, such as insulated wire, pneumatic tubing and insulation, may also be present. The aggregate effect of exposing all of these materials within a plenum is usually not known.